Re: [Qemu-devel] [PATCH v4 4/6] introduce new vma archive format

[Dietmar Maurer]

> > > We track zero regions at 4K level, and Cluster size is 64K.
> > >
> > > So I normally just use 1bit to store information about empty blocks.
> > >
> > > I thought that is quite good, but you obviously have a better idea?
> >
> > Maybe I didn't understand right then. These zero regions are just for
> > describing empty sectors within non-empty clusters?
> 
> I currently also write an entry for empty sectors. But this is only done as
> additional error check, to prove that the backup tool wrote all data.

BTW, this can be also useful when you have a damaged backup file. The extend 
header
contains checksum and special markers. All that info together let you know
what blocks are missing (exactly).